Pelvic Lipoma: A Rare Cause of Occult and Progressive Obstructive Nephropathy in a Transplanted Kidney

Background: Pelvic lipoma leading to occult obstruction as a late failure in a kidney graft (KG) is an extremely rare and challenging diagnosis. The Case: A 38-year-old man with a KG for 23 years presented with progressive fluid overload and renal failure. Pelvic ultrasound revealed an 8 cm echogenic KG that lacked significant hydronephrosis. However, significant fat was seen surrounding its pelvis and ureter. Computerized tomography disclosed a pelvic mass with homogeneous fatty content and a well-defined capsule. The mass lacked hypermetabolism on PET scanning, and the KG function was poor on MAG III estimate. The mass, with its intact capsule and its encapsulated KG, was removed. Its biopsy showed rounded fat cells that lacked inflammation, fibrosis, granulomata, muscle, vascular proliferation, and mitosis. Biopsy of the KG showed end-stage features. For the past 2 years, the patient has been stable on maintenance hemodialysis and is awaiting future kidney transplantation. Conclusion: Pelvic lipoma can lead to occult and progressive KG loss due to chronic obstruction, without hydronephrosis, simulating chronic rejection. Its differential diagnosis includes liposarcoma, lipomatosis, teratoma/dermoid cysts, and granulomatous disease. Its removal with an intact capsule is essential to avoid fat leak and future recurrence.